X-ray alignment system for fabricaing electronic chips

ABSTRACT

A method of using an x-ray alignment method by replacing a Vision Camera with a commercial x-ray sub-assembly in a chip fabricating assembly. The invention aligns the gold plated receptor pads of five mils or less, disposed upon a low light contrast ceramic (9011 alumina) substrate, with a screen printing stencil having very small apertures of less than 125 microns.

BACKGROUND OF THE INVENTION

[0001] The screen printing of very fine pitch dimensions for electronic (Flip) chips is becoming more common in the art. Chip site fiducials are commonly used for alignment of the stencil mask to the actual part to be printed. Recently, the densities of these chips has increased dramatically such that these fiducials cannot be added to the design because they utilize too much space on the substrate. This has resulted in the use of the Flip Chip pads for alignment purposes. However, this procedure presents problems with sizing and lighting. Screen printers now use a Vision System camera for aligning the stencil to the pad. The Vision System camera, however, is limited to approximately twenty mils minimum for feature size. Therefore, C4 pads whose size is 5 mils or less, cannot be used as alignment targets. This problem is further complicated by the fact that ceramic substrates present very little contrast with the pads. Therefore, screen printing is limited to high contrast, large feature substrate applications.

[0002] In lithography, visible light is employed to perform measurements in alignment systems. However, when finer measurements are to be made, x-ray technology can be employed. Because of their small wavelengths, x-rays can provide optical performance beyond the limits of conventional optical parameters.

[0003] 1. Field of the Invention

[0004] This invention relates to the fabrication of electronic chips and, more particularly, to an x-ray lithography and metrology system that aligns the stencil mask to the part to be printed upon the electronic chip.

[0005] 2. Discussion of Related Art

[0006] In U.S. Pat. No. 4,016,416 issued to Shepherd et al for “Phase Compensated Zone Plate Photodetector,” a zone plate with a photodetector mounted on the opposite face is illustrated.

[0007] In U.S. Pat. No. 3,984,680 issued to Smith for “Soft X-Ray Mask Alignment System,” an x-ray mask alignment system is illustrated featuring x-ray fluorescence detectors mounted upon the mask. The x-ray detectors measure the x-ray fluorescent signal, which provides a low intensity output as compared with an electron flux.

[0008] In U.S. Pat. No. 4,614,433 issued to Feldman for “Mask-to-Wafer Alignment Utilizing Zone Plates,” a mask-to-wafer alignment using zone plates illuminated by light during alignment is illustrated.

[0009] In U.S. Pat. No. 6,272,202 issued to Chiba et al on Aug. 7, 2001 for “Exposure Method and X-Ray Mask Structure for Use With the Same,” an exposure method for printing circuitry onto a silicon wafer is illustrated.

[0010] In U.S. Pat. No. 6,237,218 issued to Ogawa et al on May 29, 2001 for “Method and Apparatus for Manufacturing Multilayered Wiring Board and Multi-Layered Wiring Board,” a method of using alignment marks during the lamination steps and a specialized x-ray vision and mechanical alignment machine are illustrated for fabricating printed wire boards.

[0011] In U.S. Pat. No. 5,168,513 issued to Maldonado et al on Dec. 1, 1992 for “X-Ray Metrology and Alignment Detection System,” a process for aligning an x-ray mask and a work piece with an alignment mark is depicted. The apparatus generates and uses Kev x-rays, whereas the current invention uses Mev generated x-rays.

[0012] In Japanese Disclosure Document No. JP05-315215 issued to Koji in 1993 for “A Semiconductor Manufacturing Apparatus,” an alignment method using x-ray radiation through an aperture is shown.

[0013] In Japanese Disclosure Document No. JP62144325 issued to Shinichi on Jun. 27, 1987 for “Positioning Method,” the alignment of a wafer is shown using a fluorescent screen and a metal shielding x-ray pattern.

SUMMARY OF THE INVENTION

[0014] In accordance with the present invention, a method is provided to perform a screen printing of very fine pitch dimensions upon a chip substrate. The fine pitch dimensions and low light contrast of the ceramics used for the substrate prevents the use of fiducials to align the printing stencil. Using an x-ray alignment method, by replacing the Vision Camera with a commercially available x-ray sub-assembly, the invention is able to align the gold plated receptor pads of the low light contrast substrates with the screen printing stencil having very small apertures of less than 125 microns. The method comprises the steps of:

[0015] a) beaming x-rays at receptor pads disposed upon a substrate; and

[0016] b) aligning a printing screen with the receptor pads of the substrate in accordance with step (a) using the x-rays of step (a).

[0017] It is an object of the present invention to provide an improved method and apparatus for aligning a low contrast substrate with a fine pitch printing screen having apertures of less than 125 microns.

[0018] It is another object of this invention to provide an x-ray lithography system for aligning a work piece with a fine pitch mask.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when considered in conjunction with the subsequent detailed description, in which:

[0020]FIG. 1 illustrates perspective and detail views of the Vision Probe assembly of the prior art;

[0021]FIG. 2 depicts a plan view of a substrate having pads of 5 mils or less; FIG. 3 shows a plan view of a fine pitch stencil;

[0022]FIG. 4 illustrates a plan view of the stencil of FIG. 3 aligned with the pads depicted in FIG. 2, in accordance with this invention; and

[0023]FIG. 5 depicts a block diagram of an x-ray system that replaces the vision camera of FIG. 1, in accordance with this invention.

[0024] For purposes of brevity and clarity, like components and elements of the apparatus of this invention will bear the same designations or numbering throughout the figures.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0025] Generally speaking, the invention features a method of using an x-ray alignment method by replacing a Vision Camera with a commercial x-ray sub-assembly in a chip fabricating assembly. The invention aligns the gold plated receptor pads of five mils or less disposed upon a low light contrast ceramic (9011 alumina) substrate, with a screen printing stencil having very small apertures of less than 125 microns.

[0026] Now referring to FIG. 1, a prior art Vision Camera system 10 is shown for aligning a work piece with a stencil. The present invention proposes to replace the Vision Camera 15, shown in the detail 12 of FIG. 1, with a commercially available x-ray unit. The new x-ray assembly will be used to align the gold plated receptor pads 16 of the low light contrast substrate 18 shown in FIG. 2, with the screen printing stencil 20 having very small apertures 22 of less than 125 microns, as illustrated in FIG. 3.

[0027] Referring to FIG. 4, a plan view of the stencil of FIG. 3 is shown aligned with the pads 16 depicted in FIG. 2, in accordance with this invention.

[0028] Referring to FIG. 5, a block diagram is illustrated showing the x-ray sub-assembly system 50 that replaces the vision camera 15 depicted in FIG. 1. The sub-assembly system comprises an x-ray gun 52 that projects high speed electrons at a metal target 54 to produce an x-ray beam 56. The electrons are released from a heated filament (not shown). The high voltage generator 58 supplies the x-ray gun 52 with a high voltage, causing the electrons to accelerate. An SR2 controller 66 controls the voltage generation.

[0029] In normal air, under ambient air pressure, the electrons would collide with air molecules and would not reach the high velocities required to produce x-rays. Therefore, the system is operated under a high vacuum. The high vacuum is achieved by the use of a two stage pumping system comprising a backing pump 60, and a turbo pump 62. The electron stream is focused on the target 54 using a magnetic lens (not shown). Coils X and Y (not shown), which are fed by the XY deflection cable 64, direct the electrons to the target 54.

[0030] Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the example chosen for purposes of disclosure, and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention. Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims. 

What is claimed is:
 1. A method of aligning receptor pads of a low light contrast substrate with a fine pitch printing screen, comprising the steps of: a) beaming x-rays at receptor pads disposed upon a substrate; and b) aligning a printing screen with the receptor pads of the substrate in accordance with step (a), using the x-rays of step (a).
 2. The method in accordance with claim 1, wherein said printing screen is a fine pitch printing screen comprising apertures of less than approximately 125 mils.
 3. The method in accordance with claim 1, wherein said receptor pads comprise flip chip pads.
 4. The method in accordance with claim 1, wherein said receptor pads are gold plated.
 5. The method in accordance with claim 1, wherein said substrate is low contrast.
 6. A method of aligning receptor pads spaced no more than approximately 5 mils from one another, disposed upon a substrate, with a fine pitch printing screen having apertures of less than approximately 125 mils, comprising the steps of: a) beaming x-rays at receptor pads disposed upon a substrate; and b) aligning a printing screen with the receptor pads of the substrate in accordance with step (a), using the x-rays of step (a).
 7. The method in accordance with claim 6, wherein said receptor pads comprise flip chip pads.
 8. The method in accordance with claim 6, wherein said receptor pads are gold plated.
 9. The method in accordance with claim 6, wherein said substrate is low contrast.
 10. Apparatus for aligning receptor pads disposed upon a substrate, with a fine pitch printing screen, comprising: receptor pads disposed upon a low light contrast substrate; a fine pitch printing screen disposed adjacent said receptor pads; aligning means for aligning said receptor pads of a low light contrast substrate with said fine pitch printing screen, said aligning means comprising an x-ray system for producing x-rays that are beamed toward said fine pitch printing screen and said receptor pads.
 11. The apparatus in accordance with claim 10, wherein said printing screen is a fine pitch printing screen comprising apertures of less than approximately 125 mils.
 12. The apparatus in accordance with claim 10, wherein said receptor pads comprise flip chip pads.
 13. The apparatus in accordance with claim 10, wherein said receptor pads are gold plated.
 14. The apparatus in accordance with claim 10, wherein said receptor pads are disposed no more than approximately 5 mils from one another. 